A pacemaker is a medical device, typically implanted within a patient, which recognizes various arrhythmias such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the arrhythmias. An ICD is a device, also implantable into a patient, which additionally recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate fibrillation. Pacemakers and ICDs detect arrhythmias by sensing internal electrical cardiac signals using leads mounted within the heart. The internal signals comprise an intracardiac electrogram (IEGM). More specifically, the normal contraction of atrial heart muscle tissue appears as a P-wave within the IEGM. A sequence of consecutive P-waves defines the atrial rate. The normal contraction of ventricular muscle tissue appears as an R-wave (sometimes referred to as the “QRS complex”) within the IEGM.
Pacemakers and ICDs are often configured to be used in conjunction with a programmer that allows a physician to program the operation of the implanted device to, for example, control the specific parameters by which the device detects arrhythmia conditions and responds thereto. For example, the programmer may allow the physician to specify the sensitivity with which the implanted device senses electrical signals within the heart and to further specify the amount of electrical energy to be employed for pacing the heart in circumstances where expected heart signals are not sensed. Additionally, the programmer may be configured to receive and display a wide variety of diagnostic information detected by the implanted device, such as graphs of the IEGM sensed by the implanted device. In addition, the programmer may operate to analyze the data received from the device to assist the physician in rendering diagnoses as to possible arrhythmias and to assist the physician in programming the device to provide appropriate therapy.
Current state of the art implantable cardiac stimulation devices may have dozens or hundreds of programmable parameters that can be individually programmed using the external programmer. The programmable parameters permit the operation of the cardiac stimulation device to be tailored to the needs of the particular patient to provide optimal therapy while minimizing the risk of any unnecessary therapy. Unfortunately, it is often difficult to predict what the resultant operation will be for any given patient with any selected set of parameter settings. Hence, a potentially viable set of parameters is chosen by the physician, the implantable cardiac stimulation device is programmed using the selected set of parameters, and then the patient is sent home. Weeks or months later the patient must return to the physician's office for a follow-up appointment so that they physician may evaluate the results of the selected parameters.
Typically, the follow-up evaluation consists of the physician making judgments based upon a review of diagnostic information provided by the implanted device (including the IEGM) in combination with a 12-lead surface electrocardiogram (EKG or ECG) provided by a separate surface EKG unit. The 12-lead EKG unit provides twelve separate signals (detected using electrodes attached to different locations on the patient) that can be individually processed, displayed and reviewed or can be combined to yield a single combined surface EKG. As part of the review, the physician often needs to compare new 12-lead surface EKGs with recorded 12-lead surface EKGs from previous sessions. In any case, the physician adjusts the programming of the implanted device to improve therapy delivered to the patient. Again, the patient is sent home for several more weeks or months until another follow-up visit. This cycle may be repeated numerous times before optimal device settings are determined by the physician.
To obtain a 12-lead surface EKG, ten electrodes are manually attached the skin of the patient in the configuration shown in FIG. 1. The surface EKG derived from the ten electrodes is referred to as a “12-lead” EKG because twelve signals are derived from the ten electrodes—including signals from individual electrodes plus signals between certain pairs of the physical leads. More specifically, the ten electrodes include four limb electrodes and six “chest” electrodes. The chest electrodes are labeled: V1–V6. The limb electrodes are: RA (right arm), LA (left arm), LL (left leg) and right leg (RL), the last of which is optional. The chest electrodes provide one signal per electrode, referred to as the V1–V6 signals. The RA, LA and LL limb electrodes also provide one signal per electrode, referred to as the aVR, aVL and aVF signals (with F signifying foot as opposed to leg.) Finally, the difference between each pairing of the RA, LA and LL limb electrodes is considered a separate “lead” (referred to as the Einthoven leads I, II and III) and hence provide the last three signals of the 12-lead surface EKG. The twelve signals of the 12-lead surface EKG are summarized in TABLE I, along with the electrodes from which the signals are derived.
TABLE ISURFACE EKGPHYSICAL LEADSSIGNALSV1V1V2V2V3V3V4V4V5V5V6V6LA - RAILL - RAIILL - LAIIIRAaVRLAaVLLLaVF
It is particularly important to review the 12-lead surface EKG during follow-up sessions. See: “The Paced 12-Lead Electrocardiogram Should No Longer Be Neglected in Pacemaker Follow-Up”, by S. Serge Barold; Paul A. Levine; I. Eli Ovsyshcher, PACE 2001; 24: 1455—1458. However, the need to manually attach and remove each of the surface EKG electrodes from the patient during each follow-up session is a burden to the physician (or his or her staff) and a considerable inconvenience to the patient. In many cases, the skin of the patient must be shaved and sanded in the locations where the electrodes are to be attached to provide adequate electrical conduction. This can be quite uncomfortable and, in some cases, embarrassing for the patient. Moreover, the time required to attach and then remove the electrodes adds to the overall cost of the follow-up session. Also, from one follow-up session and another, the electrodes may not be placed at the exact same locations on the patient, thus resulting in somewhat different surface EKGs and making it more difficult for the physician to properly identify any actual differences in cardiac signals of the patient from one session to the next.
As can be appreciated, it would be desirable to eliminate the need for attaching the electrodes of the 12-lead surface EKG to patients during follow-up sessions to thereby reduce the cost and inconvenience to the patient and to eliminate problems resulting form differing electrode placement. One proposed solution is to emulate the surface EKG using internal electrical cardiac signals sensed by the implanted device so that, during a follow-up session, a separate surface EKG system is not required and external electrodes need not be attached to the patient. One technique for emulating a surface EKG using internal electrical signals is described in U.S. Pat. No. 5,740,811 to Hedberg et al., entitled “Device and Method for Generating a Synthesized ECG”, which is incorporated by reference herein. With the technique of Hedberg et al., a neural network is employed to convert electrical signals derived from implanted electrodes into a single emulated or “synthesized” surface EKG.
The technique of Hedberg et al. achieves significant improvement by eliminating the need to use a separate surface EKG unit during follow-up sessions. However, there is considerable room for further improvement. The neural network technique of Hedberg et al. appears to emulate only a single combined EKG and does not emulate each of the twelve individual signals of the conventional 12-lead EKG. Hence, the physician cannot review the individual signals nor use any hardware or software adapted for separately processing the individual signals. Accordingly, it would be desirable to provide a surface EKG emulation technique that separately emulates each of the twelve signals of a conventional 12-lead EKG and it is to this end that aspects of the invention are directed.
In addition, the technique of Hedberg et al. does not appear to take into account factors affecting the relative locations of implanted electrodes during the emulation. In particular, respiration, posture and the beating of the various chambers of the heart all affect the relative locations of internal electrodes—both with respect to one another and with respect to the locations of surface electrodes of the EKG being emulated—and hence affect the accuracy of EKG emulation. Respiration causes the heart to twist slightly thus changing the relative locations of electrodes mounted within the heart, particularly with respect to the location of the device can. The beating of the various chambers of the heart during different phases of a cardiac cycle also change the relative locations of the electrodes. Differences in overall patient posture (i.e. whether the patient is sitting, standing, or lying down) also affect the location of the device can and the location of the heart and another internal organs and hence affect the relative locations of the internal electrodes. Without taking these factors into account, precise EKG emulation is not achieved. Accordingly, it would be desirable to provide a surface EKG emulation technique that takes into account factors affecting the relative locations of internal electrodes and it is to this end that aspects of the invention are directed.
Still other aspects of the invention are directed to providing a calibration technique for calibrating the surface EKG emulation for use with a particular patient and a verification technique for automatically verifying the reliability of the surface EKG emulation. Any significant change in the reliability of the surface EKG emulation is likely caused by lead dislodgment. Hence, the invention also provides a technique for automatically detecting possible lead dislodgment.